Sheets of material are often used in various industries and in a variety of ways. These materials can include paper, plastic, and other materials manufactured or processed in webs or sheets. As a particular example, long sheets of paper or other materials can be manufactured and collected in reels. These sheets of material are often manufactured or processed at a high rate of speed, such as speeds up to one hundred kilometers per hour or more.
It is often necessary or desirable to measure one or more properties of a sheet of material as the sheet is being manufactured or processed. For example, in a paper sheet-making process, it is often desirable to measure the properties of the sheet (such as its basis weight, moisture, color, or caliper/thickness) to verify whether the sheet is within certain specifications. Adjustments can then be made to the sheet-making process to ensure the sheet properties are within the desired range(s).
Some measurements may require a particular geometry of the measured sheet relative to a sensor. For example, a sensor may be required to take measurements perpendicular to the sheet. Deviations from the expected or required geometry may introduce bias, uncertainty, or other error in the measurements. This problem becomes more pronounced when taking measurements of a moving sheet, which may flutter or otherwise move as it passes by or between sensors.
Several techniques have been developed to take measurements of the properties of moving sheets. In one approach, rollers are placed on both sides of a sensor in the hope that a sheet would remain relatively stable between the rollers. However, this approach may increase the tension on the sheet, which may increase the likelihood of a sheet breaking during the manufacturing or other process. Also, this approach may not work well when the sheet travels at high speeds.
In another approach, a sheet is held against a suction plate that forms part of a sensor carriage or that is located immediately upstream of a sensor carriage. However, this approach requires the sheet to be held in contact with the suction plate while the sheet is moving, which may increase the frictional drag and the tension on the sheet. Also, the suction plate typically has many holes and therefore many edges that contact the sheet, which could (among other things) damage the sheet surface or printing formed on the sheet.
In a third approach, a vortical air flow is generated in a small annulus with a vortex axis perpendicular to a sensor carriage surface. This helps to constrain the position of a sheet relative to the sensor carriage at the center of the annulus. However, the vortical air flow typically does not constrain the sheet position away from the center of the annular flow, which often causes aplanar curvature of the sheet in a region surrounding the center of the vortical flow.
In a fourth approach, a step is formed in a sensor carriage surface, and an air flow is introduced near the step. This forms a captive vortex in the step with a vortex axis parallel to the step. As a result, a sheet position is constrained at a location immediately following the captive vortex. However, this approach typically introduces curvature into the sheet and often allows the sheet position to be controlled only in a small area.